text: from mypy.types import CallableType, Type
  from typing import Callable, Optional
  from mypy.plugin import MethodContext, Plugin
  
  
  class MethodDecoratorPlugin(Plugin):
      def get_method_hook(self, fullname: str) -> Optional[Callable[[MethodContext], Type]]:
          if 'Foo.a' in fullname:
              return method_decorator_callback
          return None
  
  def method_decorator_callback(ctx: MethodContext) -> Type:
      if isinstance(ctx.default_return_type, CallableType):
          str_type = ctx.api.named_generic_type('builtins.str', [])
          return ctx.default_return_type.copy_modified(ret_type=str_type)
      return ctx.default_return_type
  
  def plugin(version):
      return MethodDecoratorPlugin
  